Although Reynolds was selected in the 6th round of the 1979 amateur draft by the San Diego Padres on June 5, he elected not to sign and played college baseball at Cañada College in Redwood City, California. In the 1980 draft on June 3, Reynolds was selected in the 1st round (2nd pick) of the amateur draft (Secondary Phase) by the Seattle Mariners.

On June 1, 2013 Reynolds was inducted into the Cañada College Hall of Fame and was presented with the "Colts Lifetime Achievement Award".

Reynolds spent several seasons in the minor leagues, playing for the Wausau Timbers (A) in Wisconsin in 1981, Lynn Sailors (AA) in Massachusetts in 1982, and Salt Lake Gulls (AAA) in Utah in 1983,[4] prior to his major league debut on September 2, 1983. The following season, he played AAA ball in Salt Lake before being called up again in September 1984. The 1985 season was his official rookie season in Major League Baseball when he played for the Seattle Mariners.

In 1991, Reynolds won the Roberto Clemente Award, given annually to a Major League Baseball player selected for his character and charitable contributions to his community.

On Oct. 26, 1992, Reynolds was granted free agency and signed with the Baltimore Orioles that December. After one season with the Orioles, he again entered free agency on Oct. 29, 1993. Reynolds signed with the San Diego Padres on Jan. 28, 1994, before being traded to the California Angels that March for Hilly Hathaway. The 1994 season was Reynolds' final season in the major leagues.

On July 24, 2006, Harold Reynolds was fired from ESPN. The ESPN spokeswoman confirmed that Reynolds "is no longer with the network" but did not give a reason for the departure.[5] "Three people who work at ESPN and familiar with the case said the cause was a pattern of sexual harassment."[6] Reynolds called this incident "a total misunderstanding" and that "I gave a woman a hug and I felt like it was misinterpreted."[7]

It was announced on October 30, 2006, that Reynolds planned to sue ESPN after having tried "everything possible to handle this situation quietly behind the scenes", while stating that he is seeking the money owed to him under the remainder of his contract, including interest and lost earnings.[8] The Smoking Gun obtained a copy of Reynolds' contract that was filed as part of the lawsuit. Reynolds' lawsuit is for $5 million, roughly equivalent to the value of the contract Reynolds signed that was scheduled to cover the 2006–2011 seasons.[9]

On June 11, 2007, Reynolds officially joined MLB.com as a baseball commentator.[10] Reynolds settled his lawsuit with ESPN on April 16, 2008. Nine days later, Reynolds officially joined Mets pre-game and post-game coverage on SportsNet New York as a baseball commentator.[11] Reynolds also worked with TBS on their Sunday baseball telecasts, as well as the 2008 MLB Playoffs.

Since its launch on January 1, 2009, Reynolds has been an analyst on MLB Network.[12] Reynolds regularly appears on MLB Tonight, Quick Pitch, Diamond Demo and MLB Network's breaking news and special event coverage, including the All-Star Game, Postseason and World Series.

Reynolds became a member of the MLB on Fox pregame show in 2012, which at the time was being produced out of MLB Network's studios. Reynolds worked on Fox's pregame show for two years alongside Matt Vasgersian and Kevin Millar. With the retirement of Fox's lead analyst Tim McCarver following the 2013 season, Reynolds, along with Tom Verducci, was promoted to join Joe Buck on the network's top broadcast team. Fox decided to replace Reynolds and Verducci with John Smoltz for the 2016 season.